bgmac: separate RX descriptor setup code into a new function

This cleans code a bit and will be useful when allocating buffers in
other places (like RX path, to avoid skb_copy_from_linear_data_offset).

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>